Contact protective network



July 19, 1949. -n5 2,476,842

CONTACT PROTECTIVE NETWORK Filed April 3, 1945 Fla. W"

lNl/ENTOR A. M CURTIS A TTOR/VE Y Patented July 19, 1949 UNITED STATESPATENT 'orrics CONTACT PROTECTIVE NETWORK Austen M. Curtis, SouthOrange, N. J., assignor to Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York Application April 3, 1945,Serial No. 586,375

3 Claims. (Cl.175--294) This invention relates to circuit makers andbreakers and particularly to contact protection systems.

The object of the invention is to provide pro tection for the contactsof a remotely controlled switching means against the destructive effectsproduced by extraordinary circuit operations. In certain situationswhere weight is a great consideration it has been found convenient toemploy glass sealed contact making devices controlled from a centralpoint for operatingremotely situated electrical devices such as smallmotors. An available contact maker has been found to be adequateandreliable for all ordinary service but liable to quick destructionunder extraordinary conditions as where the motor fails to operateproperly and the contacts are called upon to repeatedly open and closethe motor circuit and thus to handle the starting current which may bevery greatly in excess of the ordinary operating current. Since it isabsolutely essential that contact'failure must be avoided under evensuch extraordinary conditions contact protection must be provided tocare for even the rare and unlikely occurrence of these extraordinaryconditions.

It has long been known that a condenser in series with a resistancebridged about a contact used for breaking a current flow is an effectivemeans for killing a spark or an are produced by such action. However, asthe current to be broken becomes greater the amount of capacitynecessary to be usedalso becomes greater with the adverse result thatwith a greater amount of energy stored the conditions on the closing ofthe contacts become more intolerable. Heretofore where fast breakingcontacts as on an ordinary relay where the armature can have developedconsiderable speed before the contacts controlled thereby are actuallymoved apart, the.

problem of contact protection was not so acute and conditions could betolerated which would be extremely undesirable in the type of contactused herein where the armature itself carries the contact breakingsurface and whose initial movement is, therefore, correspondinglyslower. It has been discovered that contacts will not are if the voltagebetween them is held below a certain value for a certain time, but sincethis time is measured in microseconds the comparatively slow movement ofthe contacts of the glass enclosed devices is a limiting factor. Whilethese devices are capable of breaking the normal current, they aresubject to deterioration where through chattering or other reasons theymay be called upon 2 I to break the heavy current caused bythe dischargeof the contact protecting condenser or the heavy starting current of themotor.

The invention consists of an auxiliary back contact and an auxiliarycondenser and resistance for diverting the transient caused on theopening of a circuit and to limit the duration of air are if formed to aharmless period.

A feature-of the invention is the use of a back contact making device toprotect a front contact thereof. In accordance with this feature a backcontact which performs no other function in the circuit is employed toconnect into circuit an electrical network which will provide a path forenergy released through the opening of the circuit through the frontcontact thereof.

Another feature of the invention is a condenser-resistance contactprotecting network having a discharge path normally connected about thesaid condenser and capable of discharging the said condenser in aminimum period.

The drawing consists of a single sheet having three figures as follows:

Fig. 1 is a schematic circuit diagram showing a single relay with a backand front cantact provided with a contact protection network forming thebasis of the present invention;

Fig. 2 is a similar schematic circuit diagram showing an arrangementwhich may be used when a circuit is closed through two contacts inseries; and

Fig. 3 is a sectional view of a typical glass sealed contact element.

The contact element of Fig. 3 consists of a glass envelope i into oneend of which a tube 2 is sealed. This tube is at first used fortubulation purposes, that is, for exhausting the envelope of air and forlater filling it with an inert gas and is later sealed by filling theouter end with solder 3. Within the tube the element 2, which becomes aterminal of the device, has a magnetic element 4 welded to it as well asa spring 5 carrying a magnetic armature 6. The spring 5 is flexed so asto normally hold the armature 6 with its contact against a non-magneticelement 7 weldedto terminal 8. Another terminal 9 also sealed into theopposite end of the tube has a magnetic element l0 welded thereto. Asshown, there are air-gaps between elements 4, 6 and I0 so that if thiscontact device is inserted in a coil and thus affected magnetically, thearmature 6 will be drawn to bridge the gap between the two elements 4and I0 and thus to extend an electrical connection from the terminal 2to the terminal 9 breaking a normal electrical connection between theterminal 2 and the terminal 8.

Glass sealed contact devices or this nature are in common use and haveproved highly satisfactory in service, particularly in'remote controldevices. There are instances where it is desired to use such small andlight-weight elements (Fig. 3 is greatly enlarged) to control loadswhich have a high starting current characteristic. The contactestablished between the elements 6 and i is capable of carrying thenormal current but would be liable to destruction if it were called uponto break a current of greater intensity.

when relay contacts are used to break a direct current at voltagesexceeding about 20 volts, it is commonly observed that the contacts donot are visibly at very low currents, but as the current interrupted isincreased, arcing appears and the duration of the arcs increases as thecurrent is further increased. Eventually a current is reached at whichthe arc continues until the power is removed or the contactsaredestroyed. Observations on contacts which are protected by acapacitor and resistance shunt which is inadequate to prevent arcingalso show that, considerably before the current value which sustains anarc indefinitely is reached, the arc persists for several millisecondsafter the contacts have reached their maximum separation andextinguishes itself at a voltage little, if any, higher than that whichsustained it previously. This is of interest in showing that, if thecontacts form a transfer with a make and a break contact, the lattermight be used to modify the current conditions and extinguish the arclong before it would cease otherwise.

This may be taken advantage of in certain cases where a relay isordinarily called on to interrupta current which is strong enough tocause a harmful arc if the contacts are unprotected, but which normallyis within the range which permits the use of capacitor and resistancearc suppressor. Assume, for instance, a direct current motor whosenormal full load current is 4 amperes. With protection consisting of asuitable condenser, for example, microfarads, in series with 2.5 ohms,contacts carrying this current may be expected to open the circuitwithout arcing. If the motor is overloaded so that the current to bebroken is six amperes, the contacts will commence to arc, and at 8A thearcing might be prolonged and destroy the contacts in a singleoperation.

An attempt to increase the protection so that arcing would not occur at8A would require reducing Re to 1.25m and increasing 0 to 20 or moremicrofarads. This would greatly increase the welding eifect of thedischarge of the protection through the contacts on closing orchattering, and might very well cause the contacts to stick. In any casethe heavier protection would much increase the contact erosion undernormal operating conditions, although it would be useful only underunusual overloads.

In such a situation the protection shown in Fig. 1 may be useful andeconomical. A pair of contacts is protected for normal loads by C inseries with Re which hold the initial voltage between the openingcontacts below the minimal arc voltage (here taken as volts) long enoughto prevent an arc from forming when the contacts are opened on theirnormal current. An additional suppressor, Ca in series with R03 isconnected between the usual "make contact and an i added break or backcontact, so as to be in circuit only when the tongue or "swinger contactis at rest on the "break" contact. C1; is provided with a resistanceshunt Rn, low enough to discharge it during the time the relay isexpected to be held closed to operate the motor. A resistance of ten ormore thousand ohms would ordinarily be adequate. Where veryrapid openingand closing of the relay is expected a tungsten filament lamp will givea quicker discharge than a constant resistance.

The operation is as follows: When the relay is open both C and C5 arecharged to battery break contact, this voltage rise is checked by theaddition of Ca+Rcs to the circuit and the voltage then starts from alower value to rise to a new peak controlled by C+Ca. If C=Cn and islarge enough so that the surge does not rise to a value higher than thatof the battery while the swinger is traveling from the make" to .the"break contacts, and if Rc=RCB the charging current of CB when theswinger touches the break" contact, and the resulting erosion will be nogreater than those when the swinger touches the make contact.

When the motor here represented by the load L is heavily overloaded orstalled, and the relay opened, C+Rc no longer hold the initial voltagebetween the swinger and make contacts below the critical value, and anare forms and persists. When the swinger touches the "break" contact theconnection of the uncharged condenser CB across the arc, divertsmomentarily a large part of the current through Ca and Ron, dropping thearc voltage below that necessary to sustain it.

The are is extinguished and will not restrike, as

the distance between electrodes is now too large.

This method of operation limits the duration of an arc due to abnormalloading of the contacts to the travel time of the swinger from make tobreak contacts, which in suitable relay structures, may be considerablyless than 1 millisecond. The same improvement is not to be gained simplyby adding On to C. This would permit only a small increase in thecurrent before arcing occurred while increasing the erosion on contactclosing by doubling the energy dissipated at the first contacting areas.

The advantage of this method may be gained with a relay structure havingtwo contacts in series if it is provided with a back contact, as iscommon in the design of powerrelays. In this case the connections are asin Fig. 2. Here two relays are shown in conventional form with bothwindings H and i2 connected in parallel and their contacts in series sothat upon energization the load L will be connected in series with thesource of current through the armature and front contact of relay H andthe front contact and armature of relay ii. In this case the armature ofrelay I2 is provided with a back contact controlling the networks R1,,CB and Ron while the ordinary suppressors C and Re are connected in sucha way as to protect the contacts of both relays upon opening.

What is claimed is:

1. A network for protecting a pair of circuit breaking contactsconsisting of a conventional capacity and resistance circuit bridgedabout the said contacts, a second capacity and resistance circuit, acondenser discharge path bridged about the said capacity of said secondcircuit and means for bridging said second circuit about the first saidcircuit when the said contacts are moved apart.

2. In a circuit closing device, a pair of magnetically operated circuitclosers each operated by an electromagnet, the said eiectromagnets beingconnected in multiple to operate simultaneously, said circuit closersbeing connected in series, a conventional capacity and resistancecircuit bridged across said circuit closers to dissipate sparks on theopening of said contacts, a back contact on one of said circuit closersand a network ccnsisting of a resistance and having a condenser bridgedabout a portion of said resistance and connected between said backcontact and the other of said circuit closers to shunt said firstresistance and condenser circuit to dissipate the charge of said firstcondenser.

3. In a circuit maker and breaker, a contact protective networkconsisting of a primary conventional condenser and resistance circuitbridged about said circuit maker and breaker, a secondary 6 condenserand resistance circuit having a leak resistance bridged about saidsecondary condenser, and means operative upon the opening of saidcircuit maker and breaker for bridging said secondary circuit about saidprimary circuit to provide additional capacity to protect said air cuitmaker and breaker on the opening thereof and to discharge said primarycondenser during the open period thereof to protect said circuit makerand breaker on the closer thereof.

AUSTEN M. CURTIS.

REFERENCES CITED The following references are of record in the bio ofthis patent:

1 UNITED BTA'I'ES PATENTS Number

